(1) Field of the Invention
The present invention relates to a thermosetting resin composition having a low environmental load and a high flame retardancy.
(2) Description of the Prior Art
Thermosetting resins have widely been used as electronically insulating materials and building materials because of good insulating properties, heat resistance and strength, but these resins are usually poor in flame retardancy. Therefore, when they are used for the above use purposes, a flame retardant is often added thereto.
As the flame retardant, compounds including halogens such as bromine and chlorine and compounds including phosphorus have extensively been used. In the case of the former compounds, however, a poisonous halogen gas is generated at a fire of a resin product including the halogen or at its incineration after disposal. In the case of the latter compounds, the poisonous gas is scarcely generated at the combustion of a resin product including phosphorus, but when it is discarded for land reclamation, the poisonous phosphorus compound is easily dissolved out. Both the cases have a problem that an environmental load is serious.
On the contrary, it is known that a hydrate of an inorganic substance having a low environmental load such as a hydrate of aluminum hydroxide also has a flame retardant effect, but it is necessary to add a large amount of the hydrate to the resin. Hence, the employment of the hydrate has a bad influence on moldability and other characteristics, and for this reason, it is not general-purpose.
For the flame retardation under the low environ- mental load, it can be contemplated to investigate a crosslinking structure of a thermosetting resin composition with the intention of improving the flame retardancy of the 15 resin itself. In a usual combustion of plastics, it is accepted that the plastics are thermally decomposed to generate combustible decomposition products, and they ignite to advance the combustion (Hanabusa Kazuta, "Flame Retardation of Plastics", Nikkan Kogyo Newspaper Publishing Co., p. 39-41, Jun. 26, 1978). It has been reported that in the case of a thermosetting resin, if this resin includes an aromatic moiety in its structure or has a high crosslinking density, a ratio of residual carbon is high at the time of heating, so that thermal decomposition resistance, i.e., heat resistance is high (Tadashi Jinbo, "Epoxy Resin Handbook", Nikkan Kogyo Newspaper Publishing Co., p. 368-369, Dec. 25, 1987).
Heretofore, for the improvement of the flame retardancy, it has been taken as a main means to improve the heat resistance in such a way. Usually, for a combination of a resin which is a main material to be thermally cured and a curing agent, materials including aromatic compounds are selected, and the resin which is the main material and the curing agent are usually blended with each other without excess and lack so that the reactive functional groups of the resin as the main material and the curing agent may react in a ratio of 1:1 in terms of chemical equivalent, whereby a crosslinking density which is indexed by a glass transition temperature of a cured material can be maximized.
However, even if the aromatic compounds are used and the resin as the main material and the curing agent are blended in an equivalent ratio to heighten the crosslinking density and to thereby improve the heat resistance, the flame retardancy is often insufficient. For example, even in a combination system of a phenol novolac curing agent and a cresol novolac type epoxy resin which is one of thermosetting aromatic resins having the most excellent glass transition temperature and heat resistance, the flame retardancy is insufficient, and to such a system, a bromine compound is usually added as a flame retardant (Keishou Miyasaka, "Plastic Dictionary", Asakura Shoten Co., Ltd,, p. 273, Mar. 1, 1992).
This reason would be considered to be that the flame retardancy of the resin is evaluated by its self-extinguishing properties after ignition (a combustion test method by UL94) and its oxygen demand for combustion (an oxygen index method by JIS-K-7201) in accordance with stipulated evaluation procedures, and hence not only the heat resistance of the resin but also another factor such as the formability of a non-combustible structure typified by a graphite structure has a large influence on the evaluation.
On the other hand, as the thermosetting resin compositions, combinations of epoxy resins including a crystalline biphenyl group and naphthalene group in the structure and phenol novolac resins as the curing agents have been already investigated, but with regard to the effects of these combinations, the improvement of the crack resistance of molded products such as IC packages, and the like are merely known (Japanese Patent Application Laid-open Nos. 47725/1986 and 12417/1991), and a relation between a composition, particularly a crosslinking structure of these combinations and the flame retardancy is not known.
The flame retardants including halogens and phosphorus which have been heretofore used in order to improve the flame retardancy of the thermosetting resins have a problem that their environmental load is large, as described above.
In addition, when it is attempted to improve the flame retardancy of the resin itself without adding any flame retardant, it has heretofore been a solution means to improve the heat resistance (the thermal decomposition resistance) of the resin, but the resin having the high heat resistance does not always exhibit the sufficient flame retardancy.